let ``No.395 永遠の17歳``() =

  let a = stdin.ReadLine() |> int

  // X進表記
  match a % (a-7) = 7 , a / (a-7) = 1 with
  | true, true -> a-7
  |  _  ,  _   -> -1
  |> stdout.WriteLine

  ()
``No.395 永遠の17歳``()